During the 1920s and 1930s, trailer hitches began incorporating a hitch ball attached to an extension of the tow vehicle and a ball coupler carried by the forward end of the trailer. The coupler was engaged with the ball so that the tow vehicle could pull the trailer along the highway.
A number of developments, including improved highways, more powerful automobiles and trucks, and the advent of electromagnetic trailer brakes allowed the vehicles and attached trailer to achieve greatly increased highway speeds. These developments also permitted much heavier trailers to be towed. The traditional ball hitch was eventually found to be unacceptable for handling such increased speeds and trailer weights. In particular, the rig tended to be very unstable. The most significant source of instability was the upward hinging or toggle that occurred between the tow vehicle and the trailer at the point of the coupling ball. This was especially noticeable during braking. At such times, a portion of the linear momentum of the tow vehicle is converted to angular momentum. This causes the front of the tow vehicle to dip downwardly and the rear of the vehicle to pitch upwardly. In the case of the original ball hitch, the delay in activation of the trailer's electromagnetic brakes caused the significant weight of the trailer to push against the trailer ball. This tended to push the rear of the tow vehicle upwardly with even greater force. The height of the ball at the top of the hinge point effectively formed a toggle with the axles of the tow vehicle and trailer. Accordingly, the linear motion of the trailer was also converted into upward force on the ball. The toggle effect was exacerbated when the rig encountered bumps or dips on the highway. All of this made the traditional ball coupling highly unstable and contributed to serious highway accidents.
To counteract the instabilities of the original ball hitch, the weight-distributing hitch was introduced during the 1950s. Initially, the upward hinging exhibited between the tow vehicle and trailer was addressed by adding weight to the tongue of the trailer proximate the ball. It was determined that "a tongue weight" of approximately 10 to 15 percent of the gross trailer weight was generally adequate to prevent excessive upward hinging about the trailer ball. However, simply adding tongue weight was and still is, by itself, an inadequate remedy. This weight must be effectively distributed among the axles of the tow vehicle and the trailer so that the rear of the tow vehicle does not sag and a level, stable ride is maintained. Weight distributing trailer hitches that perform this function have been available for many years.
Notwithstanding their current widespread use, conventional weight distributing hitches still exhibit a number of disadvantages. The forces that these hitches generate to counteract the heavy tongue weight of the trailer tend to exert significant stress on the suspension of the tow vehicle. As a result, trucks, sport utility vehicles and full frame automobiles normally must be used as the tow vehicle. Smaller automobiles without a full frame are generally inadequate to support the force produced by the weight-distributing hitch. Such smaller vehicles normally cannot be used to pull a large trailer.
Moreover, the heavy tongue weights exhibited in present weight distributing hitches can create serious instabilities when the rig is driven at highway speeds. In particular, during a sudden lane change, which may result when the driver takes evasive action, the tongue weight carried by the overhang of the trailer behind the rear axle of the tow vehicle often creates a large X-axis moment transverse to the direction of travel. This moment can cause the tow vehicle and trailer to jackknife or flip. Serious accidents often result from such instability.
Conventional weight distributing trailer hitches also normally require that the trailer axles be positioned fairly close together and spaced apart a significant distance from the trailer tongue. This is needed in order to provide the trailer with the necessary tongue weight. When the trailer axles are positioned close together, optimal trailer stability may not be exhibited.
One known trailer hitch designed for use with relatively lightweight frameless automobiles employs a dolly that is mounted beneath the hitch. The dolly travels along the roadway with the rig and supports the tongue weight without distributing the weight to the tow vehicle. This apparatus is unduly complicated and introduces additional wheels that must engage the highway. A failure of the dolly wheels will render the entire rig inoperable.